Field-effect transistors (FETs) are typically produced using a standard complementary metal-oxide-semiconductor (CMOS) integrated circuit fabrication process. As is well known in the art, such a process allows a high degree of integration such that a high circuit density can be obtained with relatively few well-established masking and processing steps. A standard CMOS process is typically used to fabricate FETs that each have a gate electrode that is composed of n-type conductively doped polycrystalline silicon (polysilicon) material.
The intrinsic properties of the polysilicon gate material affect operating characteristics of the FET that is fabricated using a standard CMOS process. Silicon (monocrystalline and polycrystalline) has intrinsic properties that include a relatively small energy band gap (Eg), e.g. approximately 1.2 Volts, and a corresponding electron affinity (χ) that is relatively large, e.g. χ≈4.2 eV. For example, for p-channel FETs fabricated by a typical CMOS process, these and other material properties result in a large turn-on threshold voltage (VT) magnitude. As a result, the VT magnitude must be downwardly adjusted by doping the channel region that underlies the gate electrode of the FET. Doping to adjust the VT magnitude typically includes the ion-implantation of acceptor dopants, such as boron, through the polysilicon gate material and an underlying gate insulator into the channel region of the underlying silicon substrate. A typical VT magnitude of approximately 0.7 Volts results from the ion-implantation adjustment step.
One drawback of polysilicon gate FETs is that the VT magnitude adjustment by ion-implantation is particularly difficult to carry out in semiconductor-on-insulator (SOI) and other thin film transistor technology. In SOI technology, the FET channel region is formed in a semiconductor layer that is formed upon an insulating region of the substrate. The semiconductor layer may be only 1000 Å thick, making it difficult to obtain a sufficiently sharply defined dopant distribution through ion-implantation.
Another drawback of polysilicon gate FETs is that their intrinsic characteristics are likely to change during subsequent high temperature process steps. For example, the polysilicon gate is typically doped with boron impurities that have a high diffusivity in polysilicon. Because of this high diffusion rate, the boron impurities that are introduced into the polysilicon gate electrode of the FET diffuse through the underlying gate oxide during subsequent high temperature processing steps. As a result, the VT magnitude the FETs may change during these subsequent high temperature processing steps.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a transistor having an even lower VT magnitude, in order to operate at lower power supply voltages. There is an additional need in the art to obtain such lower VT magnitudes without using ion-implantation, particularly for thin film transistor devices in a SOI process. There is a further need in the art to obtain VT magnitudes that remain stable in spite of subsequent thermal processing steps.
Halvis et al. (U.S. Pat. No. 5,369,040) discloses a charge-coupled device (CCD) photodetector which has transparent gate MOS imaging transistors fabricated from polysilicon with the addition of up to 50% carbon, and preferably about 10% carbon, which makes the gate material more transparent to the visible portion of the energy spectrum. However, the Halvis et al. patent is directed to improving gate transmissivity to allow a greater portion of incident light in the visible spectrum to penetrate the gate. Halvis et al. did not recognize the need to improve the gate characteristics of FETs by lowering VT magnitudes or stabilizing VT magnitudes over subsequent thermal processing steps. Halvis et al. does not disclose or suggest the use of carbon in a field-effect transistor gate in the absence of incident light. Thus, the above described needs are unresolved in the art of fabrication of FETs using CMOS processes.